ar71xx: add a helper function for external LNA GPIO setup

ar71xx: add a helper function for external LNA GPIO setup

The built-in wireless MAC of the AR934x SoC can handle
external LNAs and the control signal of the LNAs can be
routed to any GPIO line. Add a helper function which
can be used to configure the GPIO lines.

The helper function will be used for AR934x boards
which are using externel LNAs to improve sensitivity.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39215 3c298f89-4303-0410-b956-a3cf2f4a3e73